1. Field of the Invention
The present invention relates to a measuring system, combined with a robot in a production line, and suitable for detecting a positional drift of a workpiece to be worked by the robot such that the robot may compensate for the drift and carry out a predetermined process.
2. Description of the Related Art
One conventional process often required in a product line is a process in which a number of non-aligned workpieces are sequentially picked up and conveyed to a next process. In order to automate the process, various methods using a robot-sensor system having a robot combined with a vision sensor have been proposed. In general, the vision sensor has a sensor head for taking an image and an image processor for controlling the sensor head and for processing the image. The sensor head is provided with a camera or, in addition, a projector emitting a pattern of light such as a slit of light and a spot of light. Although the sensor head is usually attached near an end of the robot, the sensor head may be fixed near the robot for use.
As it is not easy to realize the above system in a reliable way, in the prior art, only a part of the system was put to a practical use by setting some conditions suitably. The experimental system has been improved to a practical level and application of the system has gradually expanded. For example, a system as described in Japanese Unexamined Patent Publication (Kokai) No. 2003-34430 has been used.
However, a detected result, from such a system, is often unreliable because of a difference between workpieces to be worked or the presence a confusing characteristic shape in one workpiece which is similar to a characteristic shape to be detected in the same workpiece and is positioned near the latter shape. In other words, “No Detection (N/D)” (in which the workpiece to be detected is not detected) and/or “Misjudged Detection (M/D)” (in which the confusing characteristic shape is misjudged as the characteristic shape to be detected) may occur which may result in stopping of the production line.
As one countermeasure against the above inconvenience, a method has been proposed, as described in Japanese Unexamined Patent Publication (Kokai) No. 2001-160146, in which an alarm is transmitted or a process is changed to another process when a quality (a contrast in this case) of an image deteriorates or becomes unclear, which seems to cause N/D and/or M/D. However, as No Detection and Misjudged Detection are not caused by only the deterioration of the image quality, it is difficult to eliminate all of N/D and M/D.
By the way, as N/D may be judged easier than M/D, the stopping of the production line may be avoided by preparing an exception process including a retrial command, in case of N/D. On the other hand, as M/D cannot be judged easily, it is difficult to determine whether the exception process is necessary or not. Therefore, there is a risk that the product line is stopped by a significant problem such as a crash caused by a malfunction of the robot due to M/D.